To know absolutely that there is no God one must have infinite knowledge. But to have infinite knowledge one would have to be God. it is impossible to be God and an atheist at the same time. Atheists cannot prove that God doesn't exist.

1) How do you define God? 2) Does it really require infinite knowledge to know that God does not exist? 3) How do you know God has infinite knowledge? I can't name the fallacy off the top of my head, but these are some of the flaws in your argument.
– David BlomstromDec 30 '18 at 7:14

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To know absolutely that X one would have to be God. Therefore, people know nothing absolutely. This seems to have nothing to do with atheists in particular. We do not know absolutely even that the Sun will rise tomorrow, or 1+1=2, but they are beyond reasonable doubt. The problem is with the wrong standard of knowledge and proof, which is irrelevant to the issues discussed by humans.
– ConifoldDec 30 '18 at 7:58

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Because it is necessary to have infinite knowledge in order to be God - a premise granted for the sake of argument - it does not follow that to have infinite knowledge is sufficient to be God. God is generally credited with other attributes unconnected with infinite knowledge - omnipotence and omnibenevolence, for example.

This appears to be a form of affirming the consequent : If p then q, q therefore p. In terms of the example, if X is God then God has infinite knowledge; Y has infinite knowledge; therefore Y = X = God.

Also if the concept of God should prove to be demonstrably incoherent or self-contradictory, which is a logical possibility, one would not need infinite knowledge in order to demonstrate that God does not exist. It would be sufficient to deduce that, qua incoherent or self-contradictory, the concept of God cannot be instantiated.